During the normal process of eukaryotic cell division, chromosomes are replicated during S-phase, compacted and restructured during prophase, and aligned during metaphase. As cells subsequently transition from metaphase-to-anaphase, the replicated chromosomes are separated and segregated to each of two daughter cells. To avoid lethal errors in chromosome segregation, cells carefully regulate this metaphase to anaphase transition which would otherwise be driven irreversibly forward by the anaphase-promoting complex (APC/C), a multimeric E3 ubiquitin ligase. The APC/C drives both this transition and the subsequent process of mitotic exit by tagging specific proteins for proteolytic destruction; it promotes anaphase by facilitating the destruction of chromosomal cohesion proteins, and it promotes exit by directly targeting cyclin B and elements of the mitotic exit network. In the first phase of this project, multiple temperature-sensitive (ts) alleles of several different APC/C genes were isolated in screens for mutant C. elegans mothers that produce meiotic 1-cell arrested embryos. Analysis of these alleles revealed that: a) the APC/C is required for homolog as well as sister chromatid separation, b) identical cell cycle blocks can have distinct developmental consequence in different cell types, and c) hypomorphic mutants can be used to examine the post-metaphase functions of the APC/C. With the goal of extending these studies to include APC/C regulators, APC/C substrates, and other anaphase-related meiotic and/or cell type specific factors, the specific aims of this proposal are: 1) to pursue the molecular and functional characterization of emb-1, the single metaphase I arrested mutant that does not encode an APC/C subunit, and 2) to initiate studies of new ts mutants with defects in meiotic chromosome separation and/or segregation which nevertheless develop into multi-cellular embryos. These studies will a) identify novel and conserved players in the metaphase-to-anaphase transition, b) provide an estimate of how many genes function during this critical transition, and c) generate a unique collection of ts alleles that can and will be used in comparative studies of different types of meiotic and mitotic cell divisions.